I have similar problem. One Uno R3 does not show up in IDE or device-manager. Green power led is on. Yellow blinks a few times when reset button is pressed. It could be I have by accident shorted 5V to ground, I'm not sure. Other Uno R3 is ok, so probably not anything with setup of pc. Is the Arduino thrash?
The serial-to-usb converter has probably died or (unlikely) lost its firmware. You can try to burn the firmware again using e.g. Atmel Flip; https://support.arduino.cc/hc/en-us/articles/4408887452434-Flash-the-USB-to-serial-firmware-for-UNO-Rev3-and-earlier-and-Mega-boards
If burning the firmware again does not help, you can try using a serial-to-usb converter connected to RxD/TxD, DTR and GND.
Or you can buy a new Uno.
In future please do not hijack topics; start your own one.
ok, thanks
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